There are a few ways of making condtional statements in Japanese. In this case, it is called the "tara" form. It means when/if A happens, then B happens. The sentence construction is like this: Verb A (past tense + ra) + second Verb B (masu/mashita)

Both たら and なら are possible in this case. あなたがここで走ったら、私はあそこで走ります。 If you run here, then (after you) I will run there. あなたがここで走るなら、私はあそこで走ります。 If you will run here, I will run there. -If you use "X たらY", X should happen before Y. If you use "XならY", the order doesn't matter. X and Y can happen at the same time. 　 -'over there" is あそこ , not あこ. -The particle before 走る is either を or で , but not に.

BTW, http://www.guidetojapanese.org/learn/grammar/conditionals explains all the conditional forms (the -tara in Liv's answer, the -nara in Yamy's answer, and two more).

ありがとう！　＝D

Oh i see. If you buy this game, I will play with you→あなたがこのゲームをかうなら、いっしょにあそびます。 If you play the guitar, I will song.→あなたがギターをえんそうするなら、わたしはうたいます。